The present invention relates to a method and apparatus for reading information from a magneto-optical recording medium, such as a MAMMOS (Magnetic AMplifying Magneto-Optical System) disk, comprising a recording or storage layer and an expansion or read-out layer.
In magneto-optical storage systems, the minimum width of the recorded marks is determined by the diffraction limit, i.e. by the Numerical Aperture (NA) of the focussing lens and the laser wavelength. A reduction of the width is generally based on shorter wavelength lasers and higher NA focussing optics. During magneto-optical recording, the minimum bit length can be reduced to below the optical diffraction limit by using Laser Pulsed Magnetic Field Modulation (LP-MFM). In LP-MFM, the bit transitions are determined by the switching of the field and the temperature gradient induced by the switching of the laser. For read-out of the small crescent shaped marks recorded in this way, Magnetic Super Resolution (MSR) or Domain Expansion (DomEx) methods have to be used. These technologies are based on recording media with several magneto-static or exchange-coupled RE-TM layers. According to MSR, a read-out layer on a magneto-optical disk is arranged to mask adjacent bits during reading while, according to domain expansion, a domain in the center of a spot is expanded. Because of the advantage of the domain expansion technique over MSR, bits with a length below the diffraction limit can be detected with a similar signal-to-noise ratio (SNR) as bits with a size comparable to the diffraction limited spot. MAMMOS is a domain expansion method based on magneto-statically coupled storage and read-out layers, wherein a magnetic field modulation is used for expansion and collapse of expanded domains in the read-out layer.
Thus, in the above mentioned domain expansion techniques, like MAMMOS, a written mark from the storage layer is copied to the read-out layer upon laser heating with the aid of an external magnetic field. Due to the low coercitivity of this read-out layer, the copied mark will expand to fill the optical spot and can be detected with a saturated signal level which is independent of the mark size. Reversal of the external magnetic field collapses the expanded domain. A space in the storage layer, on the other hand, will not be copied and no expansion occurs.
The resolution of the MAMMOS read-out process, i.e. the smallest bit size that can be reproduced without interference from neighbouring bits, is limited by the spatial extent of the copy process, i.e. the so-called copy or detection window. This copy window decreases when the read-out laser power is reduced. On the other hand, a minimum laser power is required to enable the copy process. Thus, it is clear that the copy window should be as small as possible so as to reach a high storage density. This can be achieved by using a very localized, sharp temperature profile, for example, with high NA (Numerical Aperture) optics and shorter wavelength laser light. More generally speaking, successful MAMMOS read-out requires a laser power larger than the minimum laser power, while interference of neighbouring bits should be avoided as this leads to additional false MAMMOS signals, i.e. so-called xe2x80x98double peaksxe2x80x99.
FIG. 2 shows a signalling diagram of a conventional read-out strategy. The duration of the expansion direction of the external magnetic field (upper level of Hext) equals the duration of the collapse direction (lower level of Hext). The size of the copy window w determines the minimum bit length b: bmin=2 w. Since this size condition is fulfilled in FIG. 2, correct MAMMOS peaks are obtained for each mark region (upward magnetization) in the storage layer. The overlap curve indicates the amount of overlap of the copy window and a mark region. If such an overlap reaches to a preceding or subsequent expansion period of the external magnetic field within the same copy window, so-called xe2x80x9cdouble peaksxe2x80x9d are generated in the MAMMOS signal.
FIG. 3 shows a signalling diagram of the conventional read-out strategy, where the copy window is too large or the bit length b too small, such that w greater than b/2. Hence, undesired double peaks appear in the MAMMOS signal.
Thus, in general, successful MAMMOS read-out requires a laser power larger than the minimum laser power, while interference of neighbouring bits should be avoided as this leads to additional false MAMMOS signals.
If the duty cycle of the external magnetic field is adjusted so that the duration of the expansion or up direction is chosen to be smaller than the duration of the collapse or down direction, unwanted double peaks can be suppressed. As an example, the documents EP 0 913 818 A1 and EP 0 915 462 A1 suggest a smaller expansion duration and a range 0.15xe2x89xa6T1/(T1+T2)xe2x89xa60.9, wherein T1 denotes the duration of domain expansion and T2 denotes the duration of domain erasure (i.e. collapse). However, according to this suggested range, the expansion duration may as well be larger than the collapse duration.
It is an object of the present invention to provide a method and apparatus for reading from a domain expansion recording medium which enable the power margin to be optimized and the resolution improved for the same disk stack and recorder optics.
This object is achieved by a method as claimed in claim 1 and by an apparatus as claimed in claim 4.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.